1. Field of the Invention
The present invention relates to a carrier for developing electrostatic latent image formed by electrophotography and to a two-component-type developing agent using this carrier.
2. Disclosure of the Prior Art
Image-forming machines based on the electrophotographic method, such as electrostatic copier, laser printer and the like machines, use a two-component-type developing agent that contains a toner for developing electrostatic latent image on the surface of the photosensitive material and a magnetic carrier that electrically charges the toner by friction and turns in a developing device in a state of adsorbing the toner to supply the toner to the photosensitive material.
In order to prevent the adhesion of spent toner on the carrier and to adjust the charging properties, the surfaces of the carrier have generally been coated with a styrene-acrylic resin, acrylic resin, styrene resin, silicone resin, acrylic-modified silicone resin or fluorine-contained resin. As the coating resin having good property against being spent, there can be exemplified a silicone resin and a fluorine-contained resin having low surface tension. The fluorine-contained resin tends to be negatively charged and can be favorably used as a toner of the positively charging type, but easily peels off the carrier cores due to its poor bonding property and is not easy to use. The silicone resin exhibits excellent property against being spent and excellent bonding force to the carrier cores. When applied in large amounts, however, the silicone resin causes the carrier resistance to increase and the image density to decrease. When applied in small amounts, on the other hand, the silicone resin peels off after repetitively used, causing the life of the developing agent to be shortened. Thus, it is difficult to apply the silicone resin in proper amounts.
In order to improve the image quality and to extend the life of the developing agent by decreasing the scattering of toner, it is important that the toner is electrically charged by friction in amounts within a proper range without changing even after used for extended periods of time. In general, when the amount of electric charge by friction is not larger than +10 .mu.c/g, a sufficiently large image density is obtained but the toner easily separates away from the carrier and scatters to cause fogging. When the amount of charge by friction is not smaller than +20 .mu.c/g, the toner does not scatter but the image density is not sufficient.
To satisfy these requirements, there has been proposed a two-component-type developing agent of a combination of a positively charging toner and a silicone-coated carrier having an average particle diameter of from 40 to 60 .mu.M (Japanese Unexamined Patent Publication (Kokai) No. 43910/1997). However, there exists an intimate relationship between the particle diameter of the carrier and the so-called carrier dragging or the carrier-flying phenomenon. It has been known that the carrier having a particle diameter of as small as 44 .mu.m or less in the whole carrier tends to adhere onto the photosensitive material due to the electrostatic sucking force by the photosensitive material overcoming the magnetic locking force by the developing sleeve and due to a repulsive force produced by a bias voltage from the developing sleeve.
Usually, the particle diameter of the carrier is adjusted by sieving using a mesh. It is, however, difficult to remove the particles of diameters not larger than 44 .mu.m while maintaining the average particle diameter to be from 40 to 60 .mu.m. The carrier having particle diameters of from 40 to 60 .mu.m generally exhibits a low fluidity. When the developing agent having an increased toner density is used to obtain a sufficiently large image density, the fluidity of the developing agent decreases. Therefore, the replenished toner is poorly mixed and the toner is electrically charged in decreased amounts; i.e., the toner scatters and causes fogging. As the copying speed increases, the replenished toner is mixed less. Therefore, this two-component-type developing agent has not been widely used in high-speed machines.